The invention pertains to document scanners, and more specifically to methods and apparatus for determining the orientation of a scanner""s platen with respect to its sensor array prior to the scanning of a document.
In a document scanner of the type in which the invention finds utility, the scanner is conventionally provided with a platen on which a document is corner referenced for scanning. The platen""s document referencing corner defines the X-axis and the Y-axis of an orthogonal coordinate system or matrix to which the picture element image content (PEL content) of the document is referenced during scanning.
A document is illuminated during scanning by a line or footprint of light that extends in the X direction and moves in the Y direction. Light is thus reflected from the document in a line-by-line or row-by-row fashion. In the case of a document comprising a black image on white paper, high intensity light is reflected from the document""s white background PEL areas, and lower intensities of light are reflected from the document""s black image PEL areas.
The platen may contain a horizontal, rectangular, transparent glass or glass-like member on which a document is corner referenced.
The line or footprint of reflected light is optically directed onto a linear array of light sensitive cells or elements that effectively extend in the X direction. An example of such an array is a CCD array whose individual cells define one pixel or PEL of the light footprint.
The convention that will be used in the following description defines the direction in which the light footprint extends, the direction in which the sensor array extends, and the direction in which the leading edge of the platen extends, as the X direction. The direction of relative movement between the document and the light footprint is defined as the Y direction. The sides of the platen extend in the Y direction.
It is conventional to designate the edge of a document that is positioned adjacent to the platen""s leading edge as the document""s leading edge. This document edge is the first portion of the document to be scanned by the light footprint. A rectangular document includes two side edges that extend in the Y direction, and that terminate at the document""s trailing edge. While this is the usual configuration for a document scanner, the invention also finds utility in scanners that are capable of backward scanning.
A scan request may be for what is defined as a window scan. A window scan requests that a rectangular area within the body of the document be scanned. The leading edge of such a window is parallel to the document""s leading edge, and may or may not be coincident with the document""s leading edge.
When a document corner, as defined by its leading edge and one of its side edges, is positioned at one leading edge corner of the platen, the document is said to be corner referenced on the platen.
As mentioned, a document scanner can be used to scan an entire document, or only an internal portion or window thereof. In the case of a request to scan a window area within a document, the window area is defined by giving the X and Y coordinates of the window area referenced to the X-Y coordinate system that is defined by the platen.
As will be apparent, the effective (i.e. magnified) X direction length of the sensor array is somewhat greater than the X direction width of the document being scanned (for example, greater than 8xc2xd inches).
When a request is received by the scanner to scan a document, or a window within the document, the signal content of the sensor array cells is not used (i.e., not read out into memory) until the Y direction position of the light footprint has reached the leading edge of the document (or portion of the document) that is to be scanned. Also, only the length of sensor cells that actually view a document or a document window are read out into memory (i.e., less than all of the cells are read out into memory).
For many years, it was the practice in the art to accurately construct a document scanner to insure that the above operation was achieved. This relatively expensive method of scanner construction was assumed to provide a known positional relationship between the sensor array, the footprint of light, and the image content of a document being scanned.
However, in these prior scanners the construction was not always performed as designated by the engineering specifications, thus leading to scan errors. Even when the scanner was constructed to the specified tight mechanical tolerances, subsequent rough handling of the scanner, and perhaps other factors such as subsequent temperature changes, caused document scanning errors to occur.
Thus, a need existed in the art for a method and apparatus which would periodically allow the mechanical position of the sensor array and the light footprint to be electrically or logically recalibrated relative to the mechanical position of other scanner components.
This need was partially met with the method and apparatus disclosed in U.S. Pat. No. 5,144,455 to Stein et al. for a xe2x80x9cMethod and Apparatus for Locating the Document Referencing Corner in a Document Scannerxe2x80x9d. Stein et al.""s patent discloses a document scanner as described above wherein a reflection target is located at a known position relative to a platen""s document referencing corner. The reflection target is scanned prior to the scanning of a document. By scanning and then determining the location of the reflection target, the X direction physical position of a sensor array relative to a platen""s document referencing corner can be accurately determined. This determination enables a scanner to discard light readings of sensor cells which lie beyond the side edges of a platen, as well as readings of sensor cells which lie beyond the side edges of a document window. Scanning and sensing of the reflection target also enables a scanner to accurately determine the Y direction physical position of the scanner""s light footprint relative to the scanner""s document referencing corner. This determination enables a scanner to discard sensor cell light readings which are taken prior to a point in time in which the sensor array has traveled far enough in the Y direction of a platen to reach the leading edge of a document or document window.
In summary, the method and apparatus disclosed in the patent of Stein et al. enable a document scanner to accurately locate the document referencing corner of a platen. The teachings of Stein et al. are hereby incorporated by reference.
In addition to knowing where a document referencing corner is in relation to a scan line, it is desirable to know whether a platen""s leading edge is skewed with respect to a light footprint and cells of a sensor array, and whether a platen""s leading edge is orthogonal to the direction of travel of a moving light footprint and sensor array.
It is the practice in the art to make these additional determinations by (1) accurately constructing a document scanner to insure that problems with skew and orthogonality will not arise, or (2) scanning a test target placed on the platen glass and then correcting for skew and orthogonality problems using firmware, software or the like. The greatest disadvantage associated with the first of these methods is expense. When the second method is performed as a part of the manufacturing process, it too can be expensive in that there is a loss of time incurred in the placement of a test target. Furthermore, it is unlikely that an end user of a document scanner will know when (and/or take the time) to place a test target on his or her scanner for recalibration purposes.
A need therefore exists for a method and apparatus which will periodically allow the mechanical position of a sensor array and light footprint to be electrically or logically recalibrated with respect to the orientation of a scanner""s platen and other components.
The invention provides methods and apparatus for initially calibrating, or periodically recalibrating, the electronic and/or logic means of a scanner so as to ensure accurate scanning of a document, or a window within a document, without the need to rely upon a fixed and non-varying physical relationship between the sensor array, the moving footprint of light, the platen, and other elements of the scanner.
An advantage of the invention is to provide a scanner wherein the mechanical construction and alignment of various scanner members and parameters, such as the sensor array, the optical magnification of the scanning light footprint, and the position of the light path, can be constructed more easily, more economically, and to less stringent engineering specifications than was possible in the prior art.
It is a further advantage of the invention to provide a scanner which can be calibrated without the need to accurately position a test target on the scanner""s platen for calibration purposes.
While past scanner manufacturing and calibration techniques have required a scanner""s platen to be correctly oriented with respect to various scanner members and parameters, or have required the placement of a test target on a scanner""s platen so that the test target can be scanned for the purpose of making adjustments in the use and interpretation of light readings taken by a sensor array, the invention disclosed herein provides a novel construction and arrangement whereby operation of the invention accurately determines the orientation of a scanner""s platen with respect to scanner members such as the sensor array, the optical magnification of the scanning light footprint, and the position of the light path. At the same time that the orientation of the platen is determined, or closely in conjunction with said determination, the document referencing corner of the platen can also be determined, as taught by Stein et al. The orientation of the platen may be stored in memory so that recalibration of the scanner need only occur at periodic intervals rather than each time the scanner is turned on.